Skip to main content

Full text of "Text Book Of Mechanical Engineering"

See other formats


768                             Appendix I

if not made "quite clear, are calculated to confuse the student
We will, therefore, explain each method as fully as need be.

ty«                                i. Duty.—This is the oldest method.    The boiler and engine

are considered as one machine, and the number of foot-pounds of
work done are stated, as obtained from i cwt. of coal^ the best
Welsh coal being generally used. The value of this quantity
having been found from time to time for the best engines (about
60 to 100 million foot-pounds), constituted a sort of standard for
the performance of others.

2.   Coal burnt per LH.P. per hour.—This, the later standard,
also including both boiler and engine, exists more or less at the
present time.   Its value was, at one period, as much as 4, but even
with single engines it was soon lowered to 3, in two-stage com-
pounds to 2\ or 2, and in triple-stage compounds to 1*5 or even
1*3.     The connection between standards i and 2 is shewn as
follows :—

Dutv  =         112 x 33,000 x 60

y        ibs. of coal per I. H. P. hr.
a relation easily proved by simple proportion.

3.  Steam used per I. H. P. per hour.—This is a standard of
comparatively recent introduction,   and  is  adopted   where   the
engine's performance is to be gauged apart from that  of the
boiler.     The feed   water   weight   is   measured,   and   priming
eliminated as much as possible.    The value may vary from  iz\
Ibs. in extremely good cases to 30 Ibs. with ordinary working; 24
being good for non-condensing engines.

4.  Efficiency of Boiler.—Just as the weight of steam used is
often taken to represent the efficiency of the engine, so the weight
of water evaporated is often called the efficiency of the boiler.
The evaporation per Ib. of coal may be taken at 10 Ibs. in good
cases.   A better statement of boiler efficiency may be made by

!il                         separately calculating the heat units given to the water and the

heat uiiit& obtained by the combustion of i Ib..of coal.    Then,

„ ..      ~ .             Heat units given to water

Boiler efficiency =   —==--------.2_____.._.

Heat units from coal

which ri&yliave a value of |o per cent.in a good boiler. (Seep. 1004.)